A. Yu. Yakubovskii

Influence of the doping level on the charge distribution among the inequivalent CuO2 layers in Tl2Ba2Ca2Cu3O10−δ : a NMR study

Abstract 63 Cu and 17 O NMR measurements in the normal and superconducting states of Tl2Ba2Ca2Cu3O10−δ with different δ are reported. In the overdoped Tl2223 sample with Tc=117 K (Tcopt=123 K) and δ1 17 K are revealed for inequivalent CuO2 layers. For the inner CuO2 layer with the square oxygen coordination of Cu the decrease of 17 K with temperature is more gradual. In going towards the underdoped Tl2223 with Tc=104 K and δ2>δopt the changes of 63,17 K with temperature are found to be the same for both types of copper layers. The quadrupole coupling constants for copper and oxygen from different CuO2 layers were obtained. From the variations with doping of the valence contribution to the electric field gradient at copper sites, we estimate both the hole numbers at Cu and oxygen sites and the real concentration of mobile hole carriers nh in each of inequivalent CuO2 layers. In the overdoped Tl2223 sample the charge density in the inner layer differs from the one in the outer plane (with five-fold oxygen coordination for Cu). Our results show that the inhomogeneity of the charge distribution disappears in the underdoped regime. The results are compared with calculations of the charge distribution among the CuO2 planes in multilayered cuprates reported by Haines and Tallon [E.M. Haines, J.L. Tallon, Phys. Rev. B 45 (1992) 3127].

Interlayer CuO2 antiferromagnetic spin correlations in Tl-based superconducting Tl2Ba2CanCun+1O6+2n and Tl0.5Pb0.5Sr2Ca0.8Y0.2Cu2O7 oxides: 63Cu and 205Tl NMR and spin-lattice relaxation data

Abstract We analyzed 205 Tl and 63 Cu spin-lattice relaxation rate data T −1 1 for thallium-based Tl 0.5 Pb 0.5 Sr 2 Ca 0.80 Y 0.2 Cu 2 O 7 ( T ons c = 107 K) and Tl 2 Ba 2 Ca n Cu n +1 O 6+1 n ( T ons c = 104 K) ( n = 1), T ons c = 125 K ( n = 2)) cuprates which contain one or two TlO layers, respectively. The difference in temperature dependences T −1 1 ( 63 Cu) and T −1 1 ( 205 Tl) for the one TlO layer compound gives us evidence for the presence of interlayer antiferromagnetic spin correlations between the nearest copper atoms placed in the neighbouring CuO 2 layers.

Magnetic structure of low-dimensional LiCu2O2 multiferroic according to 63,65Cu and 7Li NMR studies

The complex NMR study of the magnetic structure of LiCu2O2 multiferroic has been performed. It has been shown that the spin spirals in LiCu2O2 are beyond the ab, bc, and ac crystallographic planes. The external magnetic field applied along the c axis of the crystal does not change the spatial orientation of spirals in Cu2+ chains. A magnetic field of H0 = 94 kOe applied along the a and b axes rotates the planes of spin spirals in chains, tending to orient the normal n of spirals along the external magnetic field. The rotation angle of the planes of the magnetic moments are maximal at H0 ‖ b.

73Ge NMR spectra in germanium single crystals with different isotopic composition

We have studied the influence of isotopic disorder on the local deformations in Ge single crystals from both experimental and calculation points of view. The nuclear magnetic resonance (NMR) spectra of73Ge nuclei (the nuclear spin equals 9/2) in perfect single crystals of germanium with different isotopic content were measured at temperatures 80, 300 and 450 K. Abnormal broadening of the spectrum was found to occur when the magnetic field was aligned along the [111] axis of a crystal. The observed specific angular dependence of the quadrupole broadening was attributed to isotopic disorder among atoms of germanium sited around the73Ge NMR probe. Local lattice deformations in germanium crystal lattice due to isotopic impurity atoms were calculated in the framework of the adiabatic bond charge model. The results obtained were applied to study random noncubic crystal field interactions with the nuclear quadrupole moments and corresponding effects in NMR spectra. Simulated second and fourth moments of resonance frequency distributions caused by the magnetic dipole-dipole and electric quadrupole interactions are used to analyze the lineshapes, theoretical predictions agree qualitatively with the experimental data.

Magnetic structure of the low-dimensional magnet NaCu2O2: 63,65Cu and 23Na NMR studies

The magnetic structure of a quasi-one-dimensional frustrated NaCu2O2 magnet single crystal is studied by NMR. The spatial orientation of the planar spin spirals in the copper-oxygen Cu2+-O chains is determined, and its evolution as a function of the applied magnetic field direction is analyzed.

Helical magnetic structure in a quasi-one-dimensional LiCu2O2 multiferroic crystal according to 63,65Cu NMR studies

The NMR spectra of 63Cu and 65Cu natural copper isotopes in a LiCu2O2 multiferroic single crystal compound have been measured above and below the temperature of magnetic phase transition (Tc = 23 K) in zero magnetic field and in applied magnetic field H0 = 94 kOe parallel to the c axis of the crystal. In LiCu2O2 below Tc, a complicated helical magnetic structure with the magnetic moment of copper ions Cu2+ varying along the chain according to the harmonic law with the wave vector being incommensurate to the crystal lattice constants has been revealed. The experimental results have been successfully interpreted using the model based on the planar helical magnetic structure. It has been found that the plane of rotation for Cu2+ magnetic moments in LiCu2O2 does not coincide at H0 = 0 with the ab plane. The high magnetic field (H0 = 94 kOe) applied along the c axis of the single crystal does not affect the spatial orientation of the plane of rotation.

Specific features of magnetic order in a multiferroic compound CuCrO2 determined using NMR and NQR data for 63, 65Cu nuclei

Results of studying the paramagnetic and ordered phases of a CuCrO2 single crystal using nuclear magnetic and nuclear quadrupole resonances on 63,65Cu nuclei are presented. The measurements have been carried out in wide ranges of temperature (T = 4.2–300 K) and magnetic-field strength (Н = 0–94 kOe), with the magnetic fields being directed along a and c axes of the crystal. The components of the electric-field gradient tensor and the magnetic-shift tensor (Ka,c) have been determined. The temperature dependences Ka(H || a) and Kc(H || c) for the paramagnetic phase are described by the Curie–Weiss law and reproduce the behavior of the magnetic susceptibility (χa,c). The hyperfine field on a copper nucleus has been determined, which is equal to hhfa,c= 33 kOe/μB. Below the temperature ТN = 23.6 K, nuclear magnetic resonance and nuclear quadrupole resonance spectra for 63,65Cu nuclei have been recorded typical of helical magnetic structures, which are incommensurable with the lattice period.

Development of ion-beam technique for manufacturing silicon nanowires

A new technique for manufacturing silicon nanowires on the surface of a conventional silicon wafer using ion-beam irradiation through a lithographic mask has been proposed. The conditions needed for synthesizing silicon oxide using the irradiation of the silicon substrate by protons with an energy of about 1 keV have been studied. The possibility of synthesizing silicon oxide in the region of the geometric shadow under the monocrystalline silicon nanowire has been demonstrated.

EPR observations of anomalous triplet states in Ba1−xKxBiO3 and BaPbyBi1− yO3

Electron paramagnetic resonance (EPR) spectra of samples of the systems Ba1−xKxBiO3 and BaPbyBi1−yO3 are investigated over wide ranges of composition and temperature. Two main lines in the EPR spectrum with factors g1≈2.1 and g2≈4.2 are found for all compositions. It is shown that the observed EPR line with g2≈4.2 is due to oxygen ions. This probably indicates the presence of oxygen ions with different effective charges, i.e., the existence of charge density waves in the oxygen-ion sublattice in addition to charge density waves in the bismuth sublattice.

Spin susceptibility and doping dependence in Tl0.5Pb0.5Sr2Ca1−xYxCu2O7−δ a round the metal-insulator transition 63,65Cu, 207Pb and 205Tl NMR and spin-lattice relaxation

Abstract NMR line shifts of 63Cu, 207Pb and 205Tl were measured in the cuprates Tl0.5Pb0.5Sr2Ca1−xCu2O7 (x = 0.0–1.0) in order to investigate the evolution of the spin susceptibility χS around the metal-insulator transition, which occurs when the yttrium content increases up to x ≌ 0.6. The Knight shift was evaluated for 63Cu, 207Pb and 205Tl and the corresponding change of the static spin susceptibility χS (q = 0) was determined from the NMR data in the superconducting state. In the superconducting phase, where Tc has a maximum at x = 0.2, the value of χS (q = 0) increases gradually with hole doping. The 63Cu zero-field NMR and the NMR spectra of 207Pb and 205Tl for Tl0.5Pb0.5Sr2YCu2O7 reported here present the first NMR observation of antiferromagnetic ordering in these thallium-based cuprates.